This Irish epic can be compared to Beowulf and El Cid in terms of both epicness and oldness (and the unknown-ness of who first wrote it). It comes from the Ulster Cycle of Irish stories. Maeve is a main character in this story—in fact, she's the antagonist. It's her desire for the Brown Bull that drives the attack on Ulster. (See also Maeve & Ailill vs. the People of Ulster in the "Police Reports.")
Though all of the stories in this cycle existed before 1100, the Book of Leinster is the oldest existing collection of them. Maeve is not only a regular in these stories, she's a major character. Here are a couple titles for Googling/perusing purposes: "The Adventures of Nera" and "Briccriu's Feast".
In The Dresden Files, Maeve appears as a daughter of Queen Mab, who's the Winter Queen (confusing, we know). In these books, Maeve is a fairy and not a queen but a Lady, so who knows? Maybe author Jim Butcher just really liked the name.
Celtic warrior ladies are pretty popular in the ol' comic books. N. Irish writer, Garth Ennis's series, The Boys, is a hardcore version of your normal superhero comics (instead of PG or PG-13, he tends to go for the R rating). In this series, Queen Maeve is a Wonder Woman-esque heroine who's a member of the superhero group The Seven. The story takes place in the real world, but with superheroes. So, not the real world.